1. Field of the Invention
The present invention relates to an automobile body-used light-colored water based coating composition capable of forming a multi-layer coating film showing excellent properties in yellowing resistance, anti-chipping properties and finish properties, and to a multi-layer coating film-forming method by use of the water based coating composition.
2. Description of Background Art
Particularly, an external panel of the automobile body is usually coated with a multi-layer coating film comprising an undercoat coating film with an electrodeposition coating composition, an intercoat coating film and a topcoat coating film for the purpose of imparting anti-corrosive properties and beautiful appearance. While an automobile is running, pebbles, gravels, anti-freezing agents, ice bumps, etc. on the road may rebound so as to collide against the automobile body, resulting in drawbacks of making mars on the multi-layer coating film, locally braking and peeling off the multi-layer coating film, showing poor appearance, exposing a metal substrate, developing rusts for corrosion (these drawbacks may be called as “chipping” hereinafter).
On the other hand, a coating composition using an organic solvent has mainly been used in the art. Since the use of the organic solvent may have a matter of concern about influences on health and environment due to a solvent vapor volatilized into the atmosphere, the use of a water based coating composition for use in the automobile has been advanced, and in the field of the intercoat coating composition for use in the automobile, developments of a water based intercoat coating composition imparted with various kinds of functions have been made.
Recently, in a multi-layer coating film for use in an automobile outer plate, a white pearl coating color using a mica as a luster agent is becoming a mainstream, and for the purpose of imparting anti-chipping properties, a blocked polyisocyanate curing agent is used in the intercoat coating composition, wherein a blocking agent such as methyl ethyl ketoxime, alcohol and the like is used. The use of the methyl ethyl ketoxime as the blocking agent may produce a problem of a film yellowing due to heat curing, resulting in making difficult a white pearl coating color design in a coating composition having anti-chipping properties. The use of alcohol as the blocking agent may not result yellowing, but a high dissociation temperature of the blocking agent makes it impossible to obtain a satisfactory curing properties in a temperature range of curing the automobile intercoat coating composition.
Japanese Patent Application Laid-Open No. 2000-281960 discloses a coating composition containing an emulsion prepared by water-dispersing a resin particle containing an olefin polymer and an acrylic polymer in the same particle.
Japanese Patent Application Laid-Open No. 2001-98140 discloses a water dispersion prepared by water-dispersing a block copolymer obtained by copolymerizing a polymer comprising an olefin monomer and a carboxyl group or a carboxylic anhydride group-having vinyl monomer.
A simple addition of the above emulsion or the water dispersion into a water based coating composition for the purpose of improving anti-chipping properties makes impossible to obtain a coating film showing excellent properties in yellowing resistance and anti-chipping properties, so that improvements therefor are demanded.